For example, a general headphone or earphone is worn such that a speaker portion (an output unit) which outputs sound is positioned to an ear hole.
A sound wave output from the speaker vibrates air which passes through an ear canal to vibrate (shake) a tympanic membrane.
As a result, the vibration of the tympanic membrane is transmitted to three ear ossicles (a stapes, a incus, and a malleus) and further to a cochlea, which is at the inner part of the ear ossicles.
The vibration received by the cochlea vibrates the lymph in the cochlea to stimulate hair cells. The stimulation is converted to an electric signal by the hair cells and transmitted to spiral ganglion cells in the cochlea. The electric signal is further transmitted to the cerebrum through vestibulocochlear nerves, so that sound can be recognized.
However, because the headphone or earphone of this kind plugs the ear hole and makes it difficult to recognize external sound, it may be dangerous to use it while walking.
Alternatively, a bone conduction speaker (an output unit) is known which does not plug the ear hole (see Patent Document 1)
Specifically, the bone conduction speaker transfers the vibration of sound directly to the cochlea through the skull.
Thus, it is not necessary to plug the ear hole, provided that the bone conduction speaker is worn at a position where vibration of a sound wave can be transferred directly to the cochlea through bones.
Thus, because it is possible to hear external sound, there is an advantage that it can be safely used even while walking.
There is also an advantage that a person having difficulty in hearing due to troubles as to tympanic membranes or ear ossicles can hear sound, provided that cochleas and auditory nerves are not damaged.